1. Field of the Invention
This invention relates to a ball spline in which a ball spline shaft having longitudinally extending raceway grooves is formed slidingly movably via a plurality of balls.
2. Description of the Prior Art
A ball spline formed so that an outer cylindrical member having a plurality of balls is moved linearly along a ball spline shaft has heretofore been applied to an industrial robot and a transfer machine. Such ball splines include ball splines in which the outer cylindrical members have retainers for holding a plurality of balls, and ball splines in which the outer cylindrical members have no retainers.
Japanese Patent Publication No. 61046/1991 discloses a ball spline having no retainers. This ball spline has a ball spline shaft 1 provided with a plurality of axially extending raceway grooves 2 in predetermined portions of an outer circumferential surface thereof, and an outer cylindrical body 3 provided with a bore through which the ball spline shaft 1 is slidably inserted, as shown in FIG. 12. The outer cylindrical body 3 is provided in an inner circumferential surface of the bore therein with raceway grooves 4, which are opposed to the raceway grooves 2 in the ball spline shaft 1, and return passages 5. The ball spline further has spacer rings 7 in which direction conversion passages 6 are formed, side rings 8, end seals 9 and a plurality of balls 10 fitted between the raceway grooves. The spacer rings 7, side rings 8 and end seals 9 are attached to both sides of the outer cylindrical body 3 with bolts 11. A distance between edge portions in cross section of each raceway groove 4 formed in the outer cylindrical body 3 so as to retain the balls 10 is set shorter than the diameter of the balls 10.
Japanese Utility Model Laid-Open No. 52317/1983 discloses a retainer-carrying ball spline. This ball spline is formed as shown in FIG. 13, by a ball spline shaft 1 provided with a plurality of axially extending raceway grooves 2 in predetermined portions of an outer circumferential surface thereof, an outer cylindrical body 3 provided in an inner surface thereof with raceway grooves 4 opposed to the raceway grooves 2 in the ball spline shaft 1, and inclined surface portions 12 for changing the direction of movement of the balls 10, a retainer 13 for retaining the balls 10 on the inner surface of the outer cylindrical body 3, and a plurality of balls 10. The retainer 13 is formed to a thin-walled seamless cylinder, the inner diameter of which is set larger at the portion thereof which corresponds to the balls 10 positioned in a no-load region of each return passage 5 in the retainer 13, and smaller at the portion thereof which corresponds to the balls 10 positioned in a load region of each raceway groove 4. The retainer 13 is provided with elongated bores 16 so that the balls 10 rolling in the load region project slightly. The retainer 13 is secured to the outer cylindrical body 3 by screwing nut type covering tubes 17 on both sides of an inner circumferential surface thereof.
Regarding conventional ball splines, it is difficult to form to proper shapes with a high accuracy the portions of the raceway grooves 4 which are in ball load regions formed in an outer cylindrical body 3, and direction change passages 6 formed in spacer rings 7 and side rings 8. In order to harden the portions of the raceway grooves 4 which are in ball load regions formed in the outer cylindrical body 3, the outer cylindrical body 3 as a whole is hardened, and this causes an increase in the ball spline manufacturing cost.
In the conventional ball splines, it is necessary that the raceway grooves in the outer cylindrical body 3 in which the retainer 13 is fitted firmly be hardened, and it is difficult to form in the outer cylindrical body 3 portions to which the retainer 13 is secured, this causing the cost of production of the outer cylindrical body 3 to become high.